Wide‐ranging migration and destination of early olfactory placode‐derived neurons in chick embryos

Cell migration from the olfactory placode (OP) is a well‐known phenomenon wherein various cell types, such as gonadotropin‐releasing hormone (GnRH)‐producing neurons, migrate toward the telencephalon (TEL) during early embryonic development. However, the spatial relationship between early migratory cells and the forebrain is unclear. We examined the early development of whole‐mount chick embryos to observe the three‐dimensional spatial relationship among OP‐derived migratory neurons, olfactory nerve (ON), and TEL. Migratory neurons that express highly polysialylated neural cell adhesion molecule (PSA‐NCAM) emerge from the OP and spread over a relatively wide TEL area at the Hamburger and Hamilton (HH) stage 17. Most migratory neurons form a cellular cord between the olfactory pit and rostral TEL within HH18–20. The earliest axons from the olfactory epithelium (OE) were detected along this neuronal cord using DiI‐labeling at HH21. Furthermore, a few PSA‐NCAM‐positive neurons were dispersed around the cellular cord and over the lateral TEL at HH18. A long cellular cord branch extending to the lateral TEL was transiently observed within HH18–24. These results suggest a novel migratory route of OP‐derived neurons during the early developmental stages. Following GFP vector introduction into the OP of HH13–15 embryos, labeled neurons were detected around and within the lateral TEL at HH23 and HH27. At HH36, labeled cells were observed in the rostral‐lateral TEL, including the olfactory bulb (OB) region. GFP‐labeled and calretinin‐positive neurons were detected in the OB, suggesting that early OP‐derived neurons enter the forebrain and function as interneurons in the OB.